How can I get such a key into my iPhone too, so that I can sign emails and file and such with the same private key when I'm on my phone, and my public key is valid for all such operations ? Will iCloud take care of that ? And then I want it all usable from my (multiple) email clients...
What you're thinking of are Passkeys. Which are synced. Somebody would have to write an SecurityKeyProvider that talks to the Passkey API instead.
Actually I don't think it's completely impossible. The only thing is that passkeys are origin-bound. They belong to a specific AppBundle ID or domain name. If say Secretive would add passkey support then that specific public/private keypair can't be used by another app. Though it does sync across instances of the app across devices.
Clearly I'm hazy on this stuff. But if I can export the private key from my Mac, is there any use for it on my iPhone, and any way to get it in there ?